1. Field of Invention
The present invention relates to methods, systems, apparatus and products relating to reduced stress display glass with improved shape/dimensional consistency by producing curved display glass ribbons, and in particular, curved glass ribbons formed in fusion draw machines (FDM) for the drawing of liquid crystal display (LCD) glass sheets.
2. Description of Related Art
Producing flat product glass for displays, such as LCDs, involves many challenges. A key requirement in this process is the ability to produce a very consistent product shape in large product glass plates. Typical large product glass sheets range in size up to 3.3 meters square.
Corning Incorporated has developed a process known as the fusion process (e.g., downdraw process) to form high quality thin glass sheets that can be used in a variety of devices like flat panel displays. The fusion process is the preferred technique for producing glass sheets used in flat panel displays because the glass sheets produced by this process have surfaces with superior flatness and smoothness when compared to glass sheets produced by other methods. The general fusion process is described in numerous publications, such as U.S. Pat. Nos. 3,338,696 and 3,682,609, and is well-known in the art.
One embodiment of the fusion process involves using a fusion draw machine (FDM) to form a glass sheet and then draw the glass sheet between two rolls to stretch the glass sheet to a desired thickness. A traveling anvil machine (TAM) is used to cut the glass sheet into smaller glass sheets that are sent to customers.
Residual product stress and shape can be caused in the glass sheet by a number of factors, such as the process temperature profile, the glass ribbon motion caused by the TAM and glass cutting. There are a number problems that can occur in the manufacture of liquid crystal displays whenever the residual stress of glass sheet is large or its shape is not stable.
In the fusion drawing technology, the isopipe delivers a large, thin, viscous sheet of glass melt. As the viscous sheet cools, it has a propensity to develop varying mechanical stresses—resulting for example from thermal gradients, ribbon thickness variations, residual stresses, and mechanical forces from the pulling drive systems. The ribbon has a typical width of the order of 2 meters and a length varying from 2 to 6 meters. The thickness of the ribbon is 1.1 mm or less.
With such a thin large ribbon, with normal process stresses variations within the ribbon, areas of compressive stress leads to the ribbon buckling. If the compressive stress is relatively large, then multimoded shape instability may be triggered. In such a case on a production fusion process, the ribbon shape can fluctuate. The glass product stress and shape can be affected if the ribbon shape fluctuates significantly and production must be discontinued. In severe cases, the glass ribbon within the fusion process can break from the instability.
In downflow drawing, and resulting fusion, of liquid crystal display (LCD) glass sheet, it is of critical importance that a manufacturer achieve stable production of LCD glass sheet with minimal residual stress and shape deformation at high flow density and large ribbon size. To reduce LCD panel manufacturing costs, panel makers are requiring larger and larger glass sheets, such as Gen 7, Gen 8 and beyond. As the sheet size increases, the fusion process control to maintain product shape and stress capability becomes more demanding.
In the formation of LCD glass ribbon, it is desirable to have the bow in the same direction to maintain process stability. It is also desirable for LCD customers to have glass sheets with consistent shape and stress pattern. Meeting some of these customer desires, the ribbon forming process according to the prior art design hangs a flat sheet, shown in FIG. 1, vertically in a neutral position stabilized mainly by gravity and the drive rollers. This set-up, however, has a sensitivity to ribbon buckling and motion instability with occasional production variability, for example, of cutoff system or process temperatures. In these cases, manufacturing of product is discontinued until shape stability is restored. Therefore, it would be beneficial to control the bow direction and ribbon shape in the FDM versus the prior art.
As product glass sizes increase, control of the residual stress and shape deformation become more difficult. Nevertheless, larger glass product sizes are desired, and thus it is necessary to develop new products and methods that achieve larger-size product glass having residual stress and shape deformation within acceptable ranges.